Generally, optical transmission systems may use optical modulators which perform optical modulation by using, for example, a dual polarization differential quadrature phase shift keying (DP-QPSK) scheme. In the DP-QPSK scheme, a light beam input to the optical modulator is split into two light beams. After an electrical signal is superimposed on each of these two light beams, the two light beams are combined.
To superimpose electrical signals on two split light beams, ferroelectric crystal such as lithium niobate (LiNbO3) may be used. When ferroelectric crystal is used, the electrical signals are superimposed on the light beams in waveguides inside the crystal. In order to do so, crystal having a prescribed size needs to be placed. This poses a certain limit to downsizing of the optical modulators. Accordingly, the optical modulators formed by using a semiconductor chip have been examined in recent years to implement downsized and high-efficiency optical modulators.
To combine two light beams, a polarization coupler including a polarization rotating element and a polarization combining element may be used. The polarization coupler rotates the polarization direction of one light beam, out of two light beams which travel side by side, by using a polarization rotating element such as a wave plate, and combines these two light beams whose polarization directions are perpendicular to each other by using a polarization combining element such as a polarization beam combiner (PBC) prism. Specifically, when a light beam 31 passes through a wave plate 20, the polarization direction of the light beam 31 becomes perpendicular to the polarization direction of a light beam 32 as illustrated in FIG. 11 for example. Since a PBC prism 10 has a polarized light separating film provided to reflect the polarized light beam 31 and to transmit the polarized light beam 32, the light beam 31 is reflected on the polarized light separating film and is combined with the light beam 32 which passes through the polarized light separating film.
Patent Document 1: Japanese Laid-open Patent Publication No. 05-133800
The aforementioned polarization coupler is generally configured so that the wave plate 20 is bonded to the PBC prism 10 as illustrated in FIG. 11. More specifically, the polarization rotating element is fixed to the polarization combining element with fixatives such as adhesives. In this case, the fixatives applied to a bonding surface between the polarization rotating element and the polarization combining element may overflow to the periphery of the bonding surface, and this may lead to formation of a region called a fillet.
The fillet formed of the fixatives hinders passage of light beams. Accordingly, when two light beams are combined in the aforementioned polarization coupler, incident positions of these two light beams are adjusted so that the light beams pass along the routes that circumvent the fillet. Specifically, in the configuration of the polarization coupler illustrated in FIG. 11 for example, the light beam 32 is made incident on the PBC prism 10 at a position away from the periphery of the bonding surface between the PBC prism 10 and the wave plate 20. Since the incident position of the light beam 32 is adjusted in this way, the fillet formed around the bonding surface between the PBC prism 10 and the wave plate 20 does not hinder passage of the light beam 32.
However, in the case of inputting two light beams into such a polarization coupler, it is difficult to reduce a distance between two light beams to a fixed value or less. More specifically, for example in FIG. 11, the light beam 32 is made incident on the PBC prism 10 at a position away from the periphery of the bonding surface between the PBC prism 10 and the wave plate 20. Accordingly, a fixed interval is provided between the light beam 31 and the light beam 32. As a result, when the above-described polarization coupler is applied to, for example, optical modulators, two light beams with electrical signals superimposed thereon are distanced from each other. This makes it difficult to achieve sufficient downsizing of the optical modulators. This also applies to devices other than the optical modulators. In the devices including the aforementioned polarization coupler to combine two light beams, these two light beams are placed at a certain interval. As a result, downsizing is disadvantageously limited.